Current transformers are widely used in the measurement of alternating electric current. Current transformers provide an isolated secondary current that is proportional to, and smaller than, the primary current that is being measured. The primary winding of a current transformer is connected in series with the primary current that is to be measured. A secondary winding is magnetically coupled to the primary winding by a suitable magnetic core. The secondary winding is normally connected to a load that has low impedance so that secondary current can flow freely. A secondary current is induced in the secondary winding by magnetic coupling of the primary winding to the secondary winding, the magnetic coupling being strengthened by the magnetic core that is common to both windings. The secondary current is proportionally smaller than the primary current by the turns ratio of the primary and secondary windings (not taking current transformer errors into account). The primary winding frequently consists of only one turn, which is often just a current-carrying conductor installed through an opening in the middle of the current transformer magnetic core. The secondary winding usually consists of multiple turns wrapped around the magnetic core.
In order for the secondary current generated by a current transformer to be an accurate representation of the primary current, the impedance of the secondary circuit must be kept low so that current can flow freely. The impedance of the secondary circuit is often called the xe2x80x9cburden.xe2x80x9d The burden generally includes all impedances in the loop through which the secondary current flows, including stray winding impedances, stray impedances of connecting conductors, and the impedances of any other components connected in the loop (such as current-sensing resistors and relay operating coils). In order for a current transformer to drive a secondary current through a non-zero burden, a voltage must be induced in the secondary winding. The induced voltage is proportional to secondary current and is proportional to the burden, in accordance with Ohm""s law (voltage equals current times impedance). The induced voltage is induced in the secondary winding by a fluctuating induction level in the magnetic core (the induced voltage is proportional to the rate of change of magnetic flux in accordance with Faraday""s Law). The fluctuating induction level is associated with a magnetizing current in accordance with well-known electromagnetic principles. The magnetizing current accounts for most of the error in the secondary current. Generally speaking, the accuracy of a current transformer is inversely related to the burden of the secondary circuit. A higher burden causes the secondary current to be a less accurate representation of the primary current.
FIG. 1 illustrates one prior-art configuration that is commonly used to sense alternating current. A primary current J1 flows in a primary conductor 4 which has an insulating covering 3, both of which pass through a magnetic core 1. Primary conductor 4 functions as a primary winding with only one turn. Though shown with one end disconnected, primary conductor 4 is normally connected as part of a larger electrical or electronic system. Magnetic core 1 has a secondary winding 2 wrapped around the core to form a current transformer. Winding 2 is shown with ten turns around magnetic core 1, though the actual number of turns may vary widely depending on the application. Magnetic core 1 is shown as a toroid, though wide variation in current transformer configurations is possible. Secondary winding 2 is connected to a current-sensing resistor RI having relatively low resistance to allow a secondary current J2 to flow freely. Secondary current J2 is normally smaller than primary current J1 by the turns ratio of the current transformer. Current J2 flows through resistor R1, thereby generating a voltage signal V1, which is instantaneously proportional to current J2. Voltage signal V1 is usually connected as an input to a larger current monitoring or control system (not shown). If an ideal system were possible (not having current transformer errors and other component imperfections), voltage signal V1 would be precisely instantaneously proportional to primary current J1.
The primary weakness of ordinary current transformers is their inability to measure d-c current when simply connected to a passive linear burden. While the configuration shown in FIG. 1 functions well with balanced a-c primary currents (with no d-c components), it does not function well with unbalanced currents. For example, FIG. 4A shows primary current J1 as a rectified current waveform (this is one form of pulsed d-c current). FIG. 4B shows the secondary current waveform that is typically produced by the configuration shown in FIG. 1. Times T1, T2, and T3 are included for ease of reference and comparison. The waveform is distorted and has a large xe2x80x9czero offset errorxe2x80x9d (the d-c offset error, which is most obvious whenever primary current is zero amps and secondary current is not zero amps). While the amount of distortion varies greatly between different kinds of current transformers, the large zero offset error generally does not vary much.
FIG. 2 shows a prior-art configuration that may be used to eliminate the zero offset error when measuring pulsed d-c currents. A diode D1 is added in series with resistor R1 to prevent zero offset error in the secondary current. This diode adds additional burden to the secondary circuit, thereby contributing somewhat to distortion of the secondary current waveform, as illustrated by FIG. 4C (with the waveform of FIG. 4A representing the primary current). The amount of distortion present may vary widely, depending on the particular components utilized and the magnitude of the primary current. FIG. 4D shows a typical waveform for voltage V2 (the secondary winding voltage of the configuration shown in FIG. 2). The negative part of the waveform is the sum of the diode forward voltage drop and voltage signal V1. The positive part of the waveform is the reverse voltage across the diode as the diode prevents negative current from flowing. The peak magnitude of this positive pulse increases significantly as the accuracy of the current transformer is improved (by selecting current transformers better suited to this measurement configuration). This large voltage pulse is magnetically coupled to the primary circuit, and may cause problems in applications that are sensitive to noise.
In order to measure d-c current accurately and in an isolated manner, it is common practice to use devices generally known as xe2x80x9cHall-effect current sensors.xe2x80x9d These sensors generally combine a Hall-effect sensor and a magnetic core in various ways to enable the measurement of d-c currents and a-c currents (symmetrical or unsymmetrical, with or without d-c components). Though Hall-effect current sensors are widely used, their cost is sometimes prohibitive, and many Hall-effect current sensors lack stability over time, and may therefore require frequent recalibration.
One alternative to Hall-effect current sensors is disclosed in U.S. Pat. No. 6,160,697 to Edel, issued Dec. 12, 2000. That patent describes how a voltage source (or, more generally, a xe2x80x9cvoltage devicexe2x80x9d) may be connected in the secondary circuit of an ordinary current transformer and controlled in such a way so as to control the induction level of the current transformer core. In one embodiment, the voltage source is controlled to implement a demagnetizing sequence that demagnetizes the current transformer. After such a demagnetizing sequence, the current transformer is able to measure d-c current for a limited time period (after this time period another demagnetizing sequence is required). However, secondary current is corrupted during the brief demagnetizing sequence, so d-c current cannot be measured continuously utilizing this method.
U.S. Pat. No. 6,160,697 also describes how the voltage device may be controlled to effectively reduce the burden of the entire secondary circuit to near zero ohms, thereby greatly increasing the accuracy of ordinary current transformers. This xe2x80x9cburden-reducingxe2x80x9d means is utilized in the present invention to minimize current transformer error by reducing the rate that magnetic flux builds up in the current transformer core. However, burdenreducing alone cannot prevent eventual saturation of the current transformer core when measuring d-c currents or unsymmetrical a-c currents. One embodiment of this prior-art burden-reducing circuit is shown in FIG. 3.
In FIG. 3, an op amp 6 has voltage gain controlled by variable resistor R2 and resistor R3, to form a controllable voltage device 5. Voltage signal V1 is connected as a feedback signal to voltage device 5 by a conductor 7. With this configuration, output voltage V3 is proportional to, and larger than, voltage signal V1, and has polarity so as to facilitate flow of secondary current J2. By adjusting the gain of the op amp (by adjusting variable resistor R2), the voltage drop associated with current J2 flowing through all secondary circuit resistances can be counteracted so that the effective secondary burden is greatly reduced. Secondary circuit resistances normally include winding resistance, the resistance of connecting wires, and the resistance of current-sensing resistor R1. Ground symbol 8 represents a reference potential of zero volts. As shown in FIG. 4E, the configuration shown in FIG. 3 accurately reproduces the waveform of the primary current, but still has large zero offset error.
The burden-reducing circuit of FIG. 3 compensates for the voltage drop caused by secondary current flowing through secondary circuit resistances, but does not compensate for the voltage drop associated with secondary circuit stray reactances. However, compensation for the voltage drop associated with stray reactive impedances may also be provided to reduce the effective burden of the secondary circuit still more. This may be done in several ways. First, complex impedances may be substituted for one or both of resistors R2 and R3, thereby adjusting the magnitude and phase angle of output voltage V3 as required to further reduce the effective burden (as illustrated later in FIG. 7). Alternatively, a reactance (such as an inductor) may be added in series with resistor R1 to adjust the phase angle of feedback voltage signal V1, thereby causing voltage V3 to have the same phase angle as the sum of the loop impedances. Third, some form of PID control (Proportional-plus-Integral-plus-Derivative control) may be installed in the feedback loop to improve compensation for complex stray reactances. Fourth, a digital control system may be utilized to implement almost any kind of control scheme that may be applicable, including digital PID methods and fuzzy logic.
The present invention utilizes an ordinary current transformer to provide a secondary current that is continuously proportional to a primary current; the primary current being either pulsed d-c current or pulsed a-c current (symmetrical or unsymmetrical). The most common application for the invention is thought to be in electric current measurement and control systems, providing a way to accurately measure pulsed current utilizing an ordinary current transformer to provide isolation from the primary circuit. In accordance with prior art (FIG. 3, and also U.S. Pat. No. 6,180,697), a voltage device is connected in series with the secondary winding of a current transformer. The voltage device is controlled to produce an output voltage that compensates for the voltage drop associated with secondary current flowing through secondary impedances, thereby reducing the effective burden of the secondary circuit. This sharply reduces the rate that flux builds-up in the current transformer core, thereby also reducing the rate that secondary current error increases, thereby improving the accuracy of the current transformer.
The present invention improves upon the prior art by turning the burden-reducing compensation off whenever secondary current is near zero amps, thereby providing a way for error in the secondary current to decay relatively fast and thus remain very small. The magnetic core of the current transformer is thereby also prevented from saturating, so pulsed d-c current (and pulsed a-c current) can be continuously and accurately measured.
The invention is applicable to any primary current that periodically has a magnitude of approximately zero amps for nonzero periods of time. This includes pulsed d-c current and pulsed a-c current, including unsymmetrical pulsed a-c current.
The invention may be used with sinusoidal a-c primary currents, however the ability to reproduce any d-c component that is present is lost. For sinusoidal currents, the length of time that current is at zero amps at each zero-crossing is not normally defined or measurable. When utilized with sinusoidal a-c primary currents, the invention provides accuracy improvement similar to prior-art active current transformers (without the ability to measure d-c components).
Accordingly, some objects and advantages of the present invention are:
(a) The invention provides a relatively simple way of accurately and continuously measuring pulsed d-c electric current utilizing an ordinary current transformer for isolation.
(b) The invention provides a relatively simple way of accurately and continuously measuring pulsed a-c electric current utilizing an ordinary current transformer for isolation, regardless of whether or not the a-c current is symmetrical or has an average value of zero.
Further objects and advantages will become apparent from a consideration of the drawings and the following detailed description.